Image display apparatus and high voltage driver circuit

ABSTRACT

A driver circuit of an image display apparatus comprises driver circuit with a class A/B push-pull stage (T 3 , T 5 ). The driver circuit contains an n-type pull transistor (T 3 ), an n-type control transistor (T 2 ) with a main current channel terminal coupled to a control electrode of the pull transistor (T 2 ) and a voltage source (V) applying a predetermined voltage over a series connection of the control electrode-main current channel terminals of the control transistor (T 2 ) and the pull transistor (T 3 ). The current from the control transistor (T 2 ) flows to a p-type push transistor (T 5 ) via a current mirror (T 4 , T 5 ). An input transistor (T 1 ) draws all of the current from the control transistor (T 2 ) via a node ( 142 ) between the control transistor (T 2 ) and the pull transistor (T 3 ) to control the ratio between the currents through these transistors (T 2 , T 3 ). The input of the driver circuit has a direct control over the current through the input transistor (T 1 ) that does not include a feedback through the control transistor (T 2 ).

The invention relates to an image display apparatus, a high voltagedriver circuit for driving a display screen device in such an apparatus,and more generally to a high voltage driver circuit.

A high voltage swing with high frequency is needed to drive an imagedisplay screen device such as a CRT. The output swing may be more than ahundred Volts. This makes it of prime importance to minimize the amountof current that leads to dissipated power in the driver circuit of thedisplay screen device. A well known technique to minimize the powerdissipated in driver circuits is the use of a Class A/B push pull outputstage. A class A/B push-pull output stage is an output stage that drawsa certain quiescent current, but is able to supply more output currentthan the quiescent current.

The class A/B output stage contains a push transistor connected betweenthe output and a first power supply and a pull transistor connectedbetween the output and a second power supply. Class A/B operation isrealized by driving the push and pull transistor so that the currentthrough one of the transistors can increase more than the decrease inthe current through the other transistor when the latter current nearszero. Thus, the maximum output current is higher than the quiescentcurrent. As a result a low quiescent current may be used, which leads toa small power dissipation in the output stage.

WO 96/39743 discloses an amplifier with a push-pull output stage with ann-type pull transistor and a p-type push transistor, so that thepush-pull transistors are of mutually complementary type. The p-typetransistor is arranged as the output stage of a p-type current mirror.Both the current through the n-type pull transistor and the inputcurrent of the p-type current mirror are controlled by an input voltage.A control circuit controls the currents in a complementary way so thatone current rises and the other falls when the input voltage rises.

The input current that the control circuit feeds to the p-type currentmirror can also significantly contribute to power consumption,especially when the gain of the p-type current mirror is low. To ensurelow power consumption the control circuit feeds this input current tothe p-type current mirror in such a way that little quiescent inputcurrent is needed. WO 96/39743 realizes this by using one output of along tailed pair differential amplifier to supply current to the p-typecurrent mirror. The tail current of this long-tailed pair is controlledwith a feedback loop so that the tail current increases when increasingcurrent is supplied to the p-type current mirror.

The base of a first transistor of the pair receives the input voltageand the collector of a second transistor of the pair feeds the inputcurrent to the p-type current mirror. A constant voltage V is applied tothe base of the second transistor. The feedback loop adjusts the currentIsup from the current source of the long tailed pair in proportion tothe current through the second transistor. Thus, it is ensured that onone hand the quiescent current is low and on the other hand a muchlarger current can be supplied to the input of the p-type currentmirror.

Unfortunately, the added feedback loop needed to adjust the current fromthe current source of the long tailed pair slows down the circuit. Thisfeedback loop also contains additional p-type transistors that limit thespeed of the circuit more than n-type transistors. This makes thecircuit of WO 96/39743 less useful for driving a display screen device,because high frequency operation is essential for such a device.

U.S. Pat. No. 5,038,114 shows a current amplifier that could also serveas input to a p-type current mirror and pull transistor of a push-pullstage. This amplifier contains a bipolar control transistor whose maincurrent channel could feed the input of the p-type current mirror thatcontains the push transistor of the class A/B stage. The base-emitterjunction of the control transistor is connected in series with the baseemitter junction of a bipolar n-type output transistor that could serveas pull transistor of the class A/B stage. A constant voltage is appliedacross the series connection of both junctions. The n-type outputtransistor is the output of an n-type current mirror, whose is coupledto the main current channel of the control transistor.

An input current is fed to a node between the input of the n-typecurrent mirror and the main current channel of the control transistor.Thus, the input current determines the difference between the currentthrough the control transistor and the current through the n-typecurrent mirror (which equals the current through the n-type pulltransistor). At the same time the constant voltage over the seriesconnection of the base-emitter junctions ensures that the product of thecurrent through the control transistor and the n-type output transistoris constant.

This amplifier supports class A/B operation because it makes the currentat one output go to zero when the current at the other output becomesvery large and vice versa. However, unless large input currents of bothpositive and negative polarity can be provided, this amplifier must relyfor its amplification on the amplification of the current mirrors.Therefore an amplifying p-type current mirror would have to be used inthe push-pull stage, which limits the speed of the push-pull stage. Thisalso makes this circuit less useful for driving a display screen device,because high frequency operation is essential for such a device.

Amongst others, it is an object of the invention to provide for an imagedisplay apparatus in which a display screen device can be driven at highspeed and with low power consumption.

It is another object of the invention to provide for a driver circuitthat can provide a high voltage swing at high frequencies with low powerconsumption in the driver circuit.

The invention is defined by the independent claims. The dependent claimsdefine advantageous embodiments.

The invention is based on the principle that the control transistor andthe pull transistor provide complementary currents suitable for a classAB push-pull stage when the sum of their control voltages is controlledby the voltage source. Changes in the current distribution between thecontrol transistor and the pull transistor are controlled by an externalinput signal, by drawing the changing part of the current from the maincurrent channel of the control transistor from a node between thecontrol transistor and the n-type output transistor. Substantially allof the input dependent variable part of this current (except relativelysmall currents such as for example leakage currents), and preferablyeven all of this current, is drawn from the node with the main currentchannel of an input transistor, and therefore effectively with a highimpedance current source.

The current through input transistor is controlled directly from theinput of the amplifier, i.e. not via a feedback loop through the controltransistor that affects the way the current through the input transistordepends on the input voltage, other than through the intrinsic feedbackproperties of the input transistor (such as the early effect). In thisway the speed of the amplifier is not compromised. It may be noted thatthe current through the input transistor may of course also becontrolled by a feedback loop from the output of the push-pull stage tothe input of the amplifier, in addition to control by the input voltage.Thus, the current through the input transistor may depend on the currentthrough the control transistor, but this does not detract from the factthat the input that provides the external input signal has a coupling tothe input transistor that does not include a feedback loop through thecontrol transistor, so that the control transistor does not affect thedirect relation between the current through the input transistor and theinput voltage, i.e. the dependence of the former on the latter.

Thus, the current through any one of the control transistor and then-type output transistor can be made very large, while at the same timemaking the current through the other one of the transistors very small,either by raising the current through the input transistor so as to makethe current through the control transistor large or by making thecurrent through the input transistor small, thereby inversely raisingthe current through the n-type output transistor. The current raise isnot limited by the quiescent current of the transistor that draws thisraised current (control transistor or n-type output transistor) and notslowed down by a feedback loop.

It will be appreciated that the word “transistor” as used herein refersto an area of semi-conductor substrate with a transistor function, be ita single coherent area or a combination of a number of discrete areasthat provide main current channels in parallel and that each have atransistor function.

In an embodiment the driver circuit has two functionally identicalbranches, each with an input transistor, a control transistor and a pulltransistor interconnected according to the invention, wherein the maincurrent channels of the input transistors of the two branches arecoupled to a supply connection in common via a high impedance circuitsuch as preferably a current source. Thus, the input voltages of theinput transistors in the respective branches at which a quiescent stateis realized, depend only on the difference of the input voltages, not onintrinsic properties of the circuit.

Furthermore, the main current channels of the pull transistors and thecontrol transistors in the two branches are preferably cross-connected,at least as far as they are connected to the input of the currentmirror. Thus a larger variation in the push current is made possible.Preferably the pull transistors and control transistors in both twobranches are cross-connected. Thus a minimum of current is lost.

These and other advantageous objects and aspects of the image displayapparatus and circuit according to the invention will be described inmore detail using the following Figs.

FIG. 1 shows an image display apparatus according to the invention; and

FIG. 2 shows a further image display apparatus according to theinvention

FIG. 1 shows an image display apparatus with a drive circuit 10 and adisplay screen device 12, which is a CRT. The drive circuit 10 containsa driver stage 14, a preamplifier 16 and a feedback circuit 18. Input 19a of the drive circuit 10 is coupled to an input of the preamplifier 16,which has an output coupled to an input of the driver stage 14. Anoutput of the driver stage 14 is coupled to a control electrode of theimage display device 12 and to a further input 19 b of the preamplifier16 via the feedback circuit 18. The driver stage 14 contains an npninput transistor T1, an npn control transistor T2, an npn pulltransistor T3 and a current mirror T4, T5, with a pnp mirror inputtransistor T4 and a pnp mirror output transistor T5. Input 140 of thedriver stage 14 is coupled to the control electrode of the inputtransistor T1. The main current channel of the input transistor T1 isconnected between a first power supply Vee and a node 142. The maincurrent channel of the control transistor T2 is coupled between the node142 and an input 144 of the current mirror T4, T5. The control electrodeof the control transistor T2 is coupled to a voltage source 148. Acontrol electrode of the pull transistor T3 is coupled to the node 142and its main current channel is coupled between the first power supplyVee and an output 146 of the drive stage. The input transistor T4 of thecurrent mirror is coupled as a diode (control electrode to main currentchannel) between the input 144 of the current mirror and a second powersupply connection Vcc. The output transistor T5 of the current mirrorhas a control electrode coupled to the input 144 of the current mirrorand its main current channel is coupled between the second power supplyconnection Vcc and the output of the drive stage 14.

In operation the drive circuit 10 receives an input signal (for examplea video signal) at input 19 a and drives a control electrode (forexample the cathode) of the image display device 12. Although the drivecircuit may drive the cathode directly, as shown, without deviating fromthe invention one or more buffer stages, such as a complementary emitterfollower stage (or source follower stage) may be added between thedriver and the control electrode of the image display device 12. Thefeedback circuit 18 ensures a well-defined gain and substantially linearbehavior.

The drive stage 14 amplifies the signal and generates the voltage swingneeded to control the image display device 12. In a typical imagedisplay device the power supply voltage difference Vcc−Vee needed torealize this is 100 Volts or more and frequency components of up to10–20 Mhz are amplified. The load formed by the image display device 12typically has a capacitive component of the order of 10 pF, which meansthat 10–100 mA of output current are needed, which, in view of the largevoltage swing, involves a considerable amount of power dissipation.

To ensure broadband gain, the gain is preferably provided by the n-typetransistors T1, T2, T3 of the drive stage and not by the current mirrorT4, T5. This is because pnp type transistors cause a lower cut-offfrequency than npn type transistors. The current mirror T4, T5preferably has a gain of about 1.

The npn transistors T1, T2, T3 provide gain as follows. The voltagesource supplies a voltage approximately equal to twice the base emitterjunction voltage needed to make a transistor significantly conductive,for silicon transistors for example 1.2–1.4 Volts. The sum of the baseemitter voltages of the control transistor T2 and the pull transistor T3is equal to the voltage V provided by voltage source 148.V=Vbe 2+Vbe 3The currents I2, I3 through these transistors depend approximatelyexponentially on the base emitter voltages:I 2=I 0 exp(Vbe 2/Vo) and I 3=I 0 exp(Vbe 3/V 0)Where “exp” is the exponential function (power of e) V0=kT/q, k beingBoltzmann's constant, T being absolute temperature and q being theelectron charge. I0 is a current factor which depends on thesemiconductor properties and is roughly proportional to emitter size. Asa resultI 2*I 3=Constant determined by VThe input transistor T1 draws a current I1 from node 142 dependent onthe control voltage Vin at its control electrode, which is directlydriven by the preamplifier 16. The current 11 substantially flowsthrough the main current channel of the control transistor T2(neglecting base currents etc.). As a resultI 2=I 1 and I 3=C/I 1The current I2 is fed to the input of the current mirror T4, T5 andthereby determines the current through the push transistor T5 to theoutput 146 The current I3 is the current through the pull transistor T3to the output. The quiescent current Iq of the circuit is the currentwhen the push transistor T5 and the pull transistor T3 draw the samecurrent (so that no net current flows to the output 146). The quiescentcurrent is the square root of C The quiescent current occurs when acertain voltage Vq is applied to the control electrode of the inputtransistor T1. When T1, T2 and T3 are all of the same size Vq is halfthe voltage V.Both the pull current I3 and the push current I2 depend exponentially onVin:I 2=Iq exp((Vin−Vq)/V 0)I 3=Iq exp(−(Vin−Vq)/V 0)By raising Vin above Vq an unlimited increase can be realized (inprinciple) in the current to the output via the push transistor T5. Atthe same time the current 13 through the pull transistor T3 is broughtincreasingly closer to zero. Conversely, by lowering Vin, the currentI3=C/I1 through the pull transistor can be raised without limitation inprinciple, while at the same time the current through the pulltransistor T5 is brought increasingly closer to zero. It will beappreciated that the value of Vq, which depends on the voltage of thevoltage source 148 is not critical: it merely serves to define thequiescent level. Of course, this operation does not depend linearly onVin, but if linear operation is required use may be made of the feedbackcircuit 18 to ensure linear operation, or a pre-processing circuit witha logarithmic I/O dependence may be used to make Vin depend in such away on its input signal that the net output current of the output stagedepends linearly on the input signal. If linear operation is not neededno such circuits are required of course.

Thus, the drive stage 14 of FIG. 1 realizes class AB operation, that is,operation where the quiescent current is lower than the maximum possibleoutput current. As can be seen from the equations, the quiescent state(when no net current flows to the output 146) occurs when Vin=Vq, thatis, when Vi=V/2 in the case that the npn transistors T1, T2, T3 areequal and the pnp current mirror T4, T5 has unity gain. In someapplications it is undesirable that the voltage level of Vin at whichthe quiescent state occurs is such an intrinsic property of the circuit.

FIG. 2 shows a driver stage in which a class A/B output stage accordingto the invention is realized with a differential input. Except for thecircuit 20 of transistors T1, T2, T3 that have already been discussed,this circuit contains a symmetric counterpart 22 of this circuit withinput transistor T1A, control transistor T2A and pull transistor T3A.The circuit 20 and its counterpart 22 form two branches 20, 22 that arecoupled as follows. The emitters of input transistor T1 and itscounterpart T1A are coupled to a further power supply terminal (notshown) via a common current source 24, which keeps the sum of thecurrents through the main current channels of these transistors T1, T1Asubstantially constant. The emitters of T3 and T3A are coupled together,but the sum of their emitter currents is not kept constant. Thecollectors of transistors T3 and T3A in the circuit 20 and itscounterpart 22 are cross-coupled to the collectors of the transistors T2and T2A in the symmetric counterpart 22 and the circuit 20 respectively.

In operation a differential input voltage controls the distribution ofthe current from the current source over input transistors T1 and T1A:IT 1=Is/(1+exp(−(V 1−V 2)/Vo))IT 1A=Is−I 1(Is is the current from the current source 24 and V1, V2 are thevoltages at the input). If one of the input transistors T1, T1A isstarved of current, this leads to a large current through the maincurrent channel of the pull transistor T3, T3A to which it is attached:$\begin{matrix}{{IT3} = {C/{IT1}}} \\{= {\left( {1 + {\exp\left( {{- \left( {{V1} - {V2}} \right)}/{Vo}} \right)}} \right)*{C/{Is}}}}\end{matrix}$A large current through the pull transistor occurs in either branch 20,22 symmetrically, when the difference between V1 and V2 becomes verypositive and very negative respectively. Due to the cross-coupling ofthe collectors of the control transistors T2, T2A and the pulltransistors T3, T3A this leads to an output current ofIoutput 2=Is/(1+exp(+(V 1−V 2)/Vo))+(1+exp(−(V 1−V 2)/Vo))*C/IsThe other output current depends in the same way on V1−V2, but with thepositions of V1 and V2 exchanged. It will be appreciated that the outputcurrents once more have the desirable property for class A/B outputstages that an in principle unlimited (exponential) increase in outputcurrent can be realized by raising V2−V1 and that this output current isnot bounded by the quiescent current (current through both branches whenboth branches 20, 22 draw the same current). The quiescent state of theoutput circuit is attained when V1=V2. The required input voltage doesnot depend on intrinsic properties of the transistors in the circuit.

It will be appreciated that to realize this effect the current source 24is used to ensure that current distribution between the inputtransistors T1, T1A does not depend appreciably on the common modevoltage of V1 and V2. That is, the high impedance of the current source24 is important for the circuit, not the exact value of the current fromthe current source 24 not its constancy. Similarly it will beappreciated that, although cross-coupled control transistors T2A, T2 andpull transistors T3, T3A are shown, such cross-coupling is notessential. For example the control transistors T2, T2A might be coupledto the power supply Vcc. This solution is less power efficient than thecircuit shown, but still serves the function of providing a class A/Bstage.

Although the circuit according to the invention has been described interms of bipolar transistors it will be appreciated that class A/Boperation can be realized similarly with MOS (IGFET) transistors, FETsin general or with combinations of bipolar transistors and FETs. Ofcourse this will mean that the equations discussed above no longerapply, but the principle that the net output current can increase ineither direction without being limited by the quiescent current stillapplies. In this case the voltage supplied by the voltage source 148 maybe altered accordingly. However, a drive stage with bipolar transistorsis preferred, because it allows the strongest output currents.

Similarly, it will be appreciated that the same effect can be realizedwhen not all transistors of the circuit have the same size or whenseveral transistors in parallel are used to realize the function of asingle transistor. Also it will be appreciated that without deviatingfrom the invention transistors may be added to the circuit, for examplewith their main current channel in series with the collector of thevarious transistors. This may be done to reduce the effect of dependenceof the main transistor current on the collector voltage. Suchtransistors do not affect the invention as long as the input transistorT1 or transistors T1, T1A are arranged so that a control voltage at itsor their inputs can cause an increase in both the push and pull outputcurrent that is not limited by the quiescent current, by drawing acurrent from main current channel of the control transistor T2, T2A thatdisturbs the ratio between the currents through main current channels ofthe control transistor T2, T2A and the pull transistor T3, T3A.

As a result a drive stage is realized that is capable of operating withthe high supply voltages that are needed for controlling the displayscreen device 12 (for example CRT) with a large bandwidth, will at thesame time minimizing the dissipated power, which is necessary forincorporating the circuit in an integrated circuit.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. For example, the claims explicitlyalso cover the situation in which all n-type transistors are replaced byp-type transistors and in which all p-type transistors and currentmirrors are replaced by n-type transistors and current mirrors,respectively. In the claims, any reference signs placed betweenparentheses shall not be construed as limiting the claim. The word“comprising” does not exclude the presence of elements or steps otherthan those listed in a claim. The word “a” or “an” preceding an elementdoes not exclude the presence of a plurality of such elements. The merefact that certain measures are recited in mutually different dependentclaims does not indicate that a combination of these measures cannot beused to advantage.

1. An image display apparatus, comprising: a display screen device witha drive input; and a driver circuit comprising a push-pull stage with anoutput coupled to the drive input, the driver circuit comprising: ann-type pull transistor with a main current channel coupled to the driveinput; an n-type control transistor with a main current channel terminalcoupled to a control electrode of the pull transistor via a node; avoltage source for applying a predetermined voltage over a seriesconnection of the control electrode-main current channel terminals ofthe control transistor and the pull transistor; a p-type current mirrorwith an output transistor that serves as push transistor for thepush-pull stage and an input fed by the main current channel of thecontrol transistor; and an input transistor with a main current channelcoupled to said node, the input transistor having a control electrodecoupled to an input of the driver circuit, the input transistor beingarranged to draw substantially all of a variable part of the currentfrom the control transistor, an external input of the apparatus having acoupling to the control electrode of the input transistor that does notinclude the main current channel through the control transistor.
 2. Animage display apparatus according to claim 1, the driver circuitcomprising a first and a second branch, the first branch comprising thepull transistor, the control transistor and the input transistor, thesecond branch comprising a further pull transistor, a further controltransistor and a further input transistor, interconnected as the pulltransistor, the control transistor and the input transistor in the firstbranch, the driver circuit comprising a current source that connects themain current channels of the input transistor and the further inputtransistor in common to a supply voltage.
 3. An image display apparatusaccording to claim 2, wherein the main current channel of the furtherpull transistor of the second branch is coupled to the input of thecurrent mirror in parallel with the main current channel of the controltransistor of the first branch.
 4. An image display apparatus accordingto claim 2, wherein the main current channel of the further controltransistor of the second branch is coupled to the drive input inparallel with the main current channel of the pull transistor of thefirst branch.
 5. An integrated circuit with an output contact and apush-pull stage with an output coupled to the output contact, theintegrated circuit comprising: an n-type pull transistor with a maincurrent channel coupled to the output; an n-type control transistor witha main current channel terminal coupled to a control electrode of thepull transistor via a node; a voltage source for applying apredetermined voltage over a series connection of the controlelectrode-main current channel terminals of the control transistor andthe pull transistor; a p-type current mirror with an input fed by themain current channel of the control transistor and an output transistorthat serves as push transistor for the push-pull stage; and an inputtransistor with a main current output coupled to said node, the inputtransistor having a control electrode coupled to an input of the drivercircuit, arranged to control substantially all of a variable part of thecurrent through the control transistor, an external input of theapparatus having a coupling to the control electrode of the inputtransistor that does not include the main current channel through thecontrol transistor.
 6. An integrated circuit according to claim 5,comprising a first and a second branch, the first branch comprising thepull transistor, the control transistor and the input transistor, thesecond branch comprising a further pull transistor, a further controltransistor and a further input transistor, interconnected as the pulltransistor, the control transistor and the input transistor in the firstbranch, the driver circuit comprising a current source that connects themain current channels of the input transistor and the further inputtransistor in common to a supply voltage.
 7. An integrated circuitaccording to claim 6, wherein the main current channel of the furtherpull transistor of the second branch is coupled to the input of thecurrent mirror in parallel with the main current channel of the controltransistor of the first branch.
 8. A driver circuit comprising apush-pull stage comprising: an output; a pull transistor of a firstconductivity type with a main current channel coupled to the output; acontrol transistor of the first conductivity type with a main currentchannel terminal coupled to a control electrode of the pull transistorvia a node; a voltage source for applying a voltage across a seriesconnection of the control electrode-main current channel terminals ofthe control transistor and the pull transistor; a current mirror of asecond conductivity type opposite to the first conductivity type, withan input fed by the main current channel of the control transistor andan output transistor that serves as push transistor for the push-pullstage; and an input transistor with a main current output coupled tosaid node, the input transistor having a control electrode coupled to aninput of the driver circuit, arranged to control substantially all of avariable part of the current through the control transistor, an externalinput of the driver circuit having a coupling to the control electrodeof the input transistor that does not include the main current channelthrough the control transistor.
 9. A driver circuit according to claim8, comprising a first and a second branch, the first branch comprisingthe pull transistor, the control transistor and the input transistor,the second branch comprising a further pull transistor, a furthercontrol transistor and a further input transistor, interconnected as thepull transistor, the control transistor and the input transistor in thefirst branch, the driver circuit comprising a current source thatconnects the main current channels of the input transistor and thefurther input transistor in common to a supply voltage.